Device and method for on-line control of the fibre direction of a fibre web

ABSTRACT

A device and a method for on-line control of the fibre direction of a fire web ( 8 ), being manufactured from stock delivered from a headbox ( 3 ) through a discharge opening ( 11 ) defined by movable lips ( 9, 10 ), while using a fibre direction meter ( 12 ) located downstream and actuating members ( 14 ), which are allonged long the lips for regulation of the discharge opening as a response to individual control signals, each being a function of measured fibre direction values; wherein a control unit ( 13 ) receives the measured fibre direction values, calculates the control signals, and transmits these to the actuating members. According to the invention, the control unit identifies an array of fibre direction values, originating from positions in the cross direction of the fibre web which correspond to the positions of the actuating members. The control unit then compares the array of fibre direction values with an array of desired fibre direction values.

This application is a 371 of PCT/SE03/00599, filed Apr. 15, 2003, whichclaims the benefit of provisional application 60/378,979, filed May 10,2002.

FIELD OF THE INVENTION

The present invention relates to devices and methods for on-line controlof the fibre direction of a continuous fibre web in a paper or boardmachine.

BACKGROUND OF THE INVETION

The present invention relates to a device for on-line control of thefibre direction of a continuous fibre web in a paper or board machine,comprising at least one former including at least one headbox beingarranged for delivering a stock, which in the former is formed into saidfibre web, through a slice including lips which are movable in relationto each other and define a discharge opening, said device including:

-   -   a fibre direction meter arranged downstream the former for        measuring the fibre direction of the fibre web;    -   a predetermined number of actuating members, which are arranged        in predetermined positions along said lips for regulating the        discharge opening locally as a response to individual control        signals, each being a function of measured fibre direction        values; and    -   a control unit, which is arranged for receiving the measured        fibre direction values from the fibre direction meter,        calculating said control signals, and transmitting the control        signals to the actuating members.

The invention also relates to method for on-line control of the fibredirection of a continuous fibre web in a paper or board machinecomprising at least one former including at least one headbox beingarranged for delivering a stock, which in the former is formed into saidfibre web, through a slice including lips which are movable in relationto each other and define a discharge opening, said method including:

-   -   measuring the fibre direction of the fibre web by means of a        fibre direction meter arranged downstream the former;    -   calculating and transmitting individual control signals, each        being a function of measured fibre direction values, to a        predetermined number of actuating members which are arranged in        predetermined positions along the lips for regulating the        discharge opening locally as a response to the control signals.

Within the field of papermaking, it is known to professionals that thefibre direction in a finished paper sheet, i.e. the main orientation ofthe cellulose fibres in the sheet, influences the sheet properties to agreat extent. When manufacturing paper, generally, a uniformdistribution of fibre direction along the entire paper web is aimed at,i.e. that the orientation of the fibres is similar in the machine andcross directions of the paper web. For example, it is known thatproperties of board, such as flatness, stiffness, bending resistance,stretch and printability, are improved by a uniform distribution offibre direction. Accordingly, a uniform distribution of fibre directionleads to fewer rejections of, and complaints on, the finished paperproduct.

In accordance with the so-called vector theory within papermaking, theparameters which control the fibre direction are the wire speed, thedischarge velocity of the stock and the discharge direction of the stockin relation to the machine direction. It is known to arrange ameasurement system in a paper machine in order to measure the fibredirection of the paper web in the cross direction, when the paper webpasses the system. The result from such a measurement system ispresented usually as a so-called fibre orientation profile, which is adiagram illustrating how the fibre direction varies in the crossdirection of the paper web. Based upon the measured fibre direction,working staff can then reduce any variations of fibre direction by meansof adjusting the headbox of the paper machine manually, e.g. by means ofmanual adjustment of the edge valves of the headbox or the dischargeratio, i.e. the ratio of stock discharge velocity/wire speed.

This method of reducing variations of fibre direction, however, isdifficult and irrational. Firstly, said manual adjustments arecomparatively difficult to predict. Thus, a minor adjustment may resultin an uncontrolled change of the fibre direction. Secondly, it isdifficult to predict how said adjustments, alone or in combination witheach other, influence the fibre direction. Even if the working staff hasa long experience of papermaking, the adjustment methodology tends tofollow the principle “screw and see”, i.e. the working staff measuresthe fibre direction and adjusts the headbox indiscriminately in aniterative process until a sufficiently uniform distribution of fibredirection has been obtained. This adjustment method is ineffective, anda considerable time may elapse before an acceptably uniform distributionof fibre direction has been obtained, during which period themanufactured paper web runs the risk of having to be rejected.

SUMMARY OF THE INVENTION

An object of the present invention is to remedy these problems, and toprovide a device and a method which offer on-line control of the fibredirection and which, during the current paper manufacture, enable arapid and accurate reduction of variations of the fibre direction.

The device according to the invention is characterized in that:

-   -   the control unit is arranged for identifying an array of fibre        direction values {v₁ v₂ v₃ . . . v_(N)}, of the measured fibre        direction values, said fibre direction values of said array        originating from positions in the cross direction of the fibre        web which correspond to the positions of the actuating members;        that    -   the control unit is arranged for comparing the identified array        of fibre direction values with an array of desired fibre        direction values {b₁ b₂ b₃ . . . b_(N)}, by means of calculating        an array of error values, {e₁ e₂ e₃ . . . e_(N)}={b₁-v₁ b₂-v₂        b₃-v₃ . . . b_(N)-v_(N)}; and that    -   the control unit is arranged for calculating said control signal        for each actuating member as a function of a predetermined        number of said error values in accordance with

${s_{n} = {{e_{n}C_{0}} + {\sum\limits_{i = 1}^{i = J}\;{C_{i}\left( {e_{n + i} - e_{n - i}} \right)}}}},$

-   -    where J is a predetermined integral number and C₀ C₁ C₂ . . .        C_(J) are predetermined constants.

The method according to the invention is characterized in:

-   -   identifying an array of fibre direction values, {v₁ v₂ v₃ . . .        v_(N)}, of the measured fibre direction values, said fibre        direction values of said array originating from positions in the        cross direction of the fibre web which correspond to the        positions of the actuating members;    -   comparing said identified array of fibre direction values with        an array of desired fibre direction values, {b₁ b₂ b₃ . . .        b_(N)}, by means of calculating an array of error values in        accordance with {e₁ e₂ e₃ . . . e_(N)}={b₁-v₁ b₂-v₂ b₃-v₃ . . .        b_(N)-v_(N)}; and    -   calculating each of said control signals as a function of a        predetermined number of said error values in accordance with

${s_{n} = {{e_{n}C_{0}} + {\sum\limits_{i = 1}^{i = J}\;{C_{i}\left( {e_{n + i} - e_{n - i}} \right)}}}},$

-   -    where J is a predetermined integral number and C₀ C₁ C₂ . . .        C_(J) are predetermined constants.

Owing to the facts that the discharge opening at each actuating membercan be regulated locally and that the control signals are a function ofthe measured fibre direction, undesired fibre direction variations canbe substantially continuously corrected. Preferably, the control signalsare calculated by a microprocessor, being included in a control unitarranged for receiving the measured fibre direction from the fibredirection meter and for transmitting the control signals to theactuating members after the calculation. Accordingly, the control of thefibre direction takes place without any manual actions, which enables arapid and accurate control.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described further with referenceto the figures.

FIG. 1 is a schematic representation of portions of a paper machine inwhich a device according to the invention has been mounted.

FIG. 2 shows fibre orientation profiles, which illustrate how anirregular fibre orientation profile is corrected.

FIG. 3 shows a fibre orientation profile, which illustrates how thefibre direction is changed by a local reduction of the discharge openingof a headbox in the case when the discharge velocity of the stock islower than the wire speed.

FIG. 4 shows a fibre orientation profile, which illustrates how thefibre direction is changed by a local increase of the discharge openingof a headbox in the case when the discharge velocity of the stock islower than the wire speed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic representation of a board machine 1, whichcomprises a former 2, including a headbox 3 and a wire part 4, in thiscase a fourdrinier former. The wire part 4 includes a wire 5 and abreast roll 6 around which the wire 5 runs. The headbox 3 is arrangedfor delivering stock through a slice 7 to the wire part 4 in which thestock is dewatered in order to form a continuous network of fibres, i.e.a fibre web 8. The slice 7 includes two lips 9, 10 which are arrangedfor being movable in relation to each other in order to form anadjustable discharge opening 11 through which the stock passes.

Downstream the former 2, in a position where the fibres have been fixedin the formed network, a fibre direction meter 12 is arranged formeasuring the orientation of the fibres. Preferably, the fibre directionmeter 12 is located in, or downstream, the drying section (not shown) ofthe board machine 1, but in principle, it can be located anywhere alongthe run of the fibre web 8, provided that the fibres in the selectedposition have been fixed in the network. Preferably, the fibre directionmeter 12 includes a laser-camera assembly (not shown), which performs areciprocating motion in the cross direction of the fibre web 8 in orderto measure the fibre direction in the cross direction of the fibre web8. The fibre direction meter 12 is connected to a control unit 13 whichcontrols the laser-camera assembly and which receives and processes themeasured fibre direction values. A suitable meter, for example, is theone marketed by ABB AB, Sweden, under the name “AccuRay® Smart FiberOrientation Sensor”.

According to the invention, the slice 7 includes a predetermined numberN of actuating members 14 which are placed in predetermined positions,preferably uniformly distributed, for example with a distance ofapproximately 10 cm between each other, along the lips 9, 10. Eachactuating member 14 is arranged for controlling the stock flow in itsposition in relation to the stock flow in adjacent positions. This isachieved by means of each actuating member 14 setting an individualvalue for the discharge opening as a response to a control signal fromthe control unit 13. Accordingly, the actuating members 14 are connectedto the control unit 13 in order to obtain their respective controlsignals therefrom. In the embodiment according to FIG. 1, the actuatingmembers 14 are connected to the upper lip 9, which is movable, in orderto operate the upper lip 9 in relation to the lower lip 10, which isstationary, and thereby adjust the discharge opening 11 in the differentpositions. Accordingly, the upper lip 9 is yieldable to some extent, sothat different values can be set for the discharge opening 11 along thelength of the slice 7.

In the following, the method by means of which said control signals arecalculated will be described with reference to FIGS. 2-4.

The method includes the step of the fibre direction meter 12 measuringthe fibre direction in the cross direction and transmitting the measuredfibre direction values to the control unit 13. Accordingly, the measuredfibre direction values describe a fibre orientation profile in the crossdirection of the fibre web 8. The graph 15 in FIG. 2 is a graphicillustration of such a profile. From the graph 15, it is evident thatthe fibre direction in this case makes an angle with the direction oftravel of the fibre web 8, i.e. with the machine direction, which angleis approximately −7° at one edge of the fibre web 8 and increases in thecross direction of the fibre web 8 to a value of approximately 8° at theother edge. Accordingly, the fibre web 8 exhibits an irregular fibreorientation profile in this case.

From the measured fibre direction values, the control unit 13 identifiesan array of fibre direction values,

-   -   {v₁ v₂ v₃ . . . v_(N)},        which values, being angular values between the fibre direction        and the machine direction, originate from positions in the cross        direction corresponding to the positions of the actuating        members 14.

The measured fibre direction values are then compared with an array ofdesired fibre direction values,

-   -   {b₁ b₂ b₃ . . . b_(N)},        which define a desired fibre direction profile. Normally, it is        desirable that the main fibre direction coincides with the        machine direction across the entire width of the fibre web 8,        and therefore all desired fibre direction values normally are        set to be 0°, as illustrated by the graph 16 in FIG. 2. In        principle, however, also other desired fibre direction profiles        can be chosen.

The above-mentioned comparison takes place by means of the control unit13 calculating an array of error values in accordance with

-   -   {e₁ e₂ e₃ . . . e_(N)}={b₁-v₁ b₂-v₂ b₃-v₃ . . . b_(N)-v_(N)},        i.e. by means of calculating the difference between the measured        and the desired fibre direction values. In FIG. 2, the graph 17        illustrates the calculated error values. Accordingly, the error        values define an error profile which corresponds to the        correction of the fibre direction which has to be performed in        order to obtain the desired fibre direction profile.

Thereafter, the control unit 13 calculates the control signal for eachactuating member as a function of the error values. Thus, the controlsignal s to the actuating member in the position n can be writtengenerally as

-   -   s_(n)=f( . . . e_(n−1), e_(n), e_(n+1) . . . ).

According to the above-mentioned vector theory, however, it is known howa local change of the stock flow influences the fibre direction. If, forexample, the discharge velocity of the stock is lower than the wirespeed, a local reduction of the discharge opening in a certain positionn means that the fibre direction is influenced as is evident from thefibre orientation profile in FIG. 3. To the left of the position n, thefibres are turned clockwise, i.e. in a positive direction, and to theright of the position n the fibres are turned counter-clockwise, i.e. ina negative direction. In the same fashion, it is known how the fibredirection is influenced by a local increase of the discharge opening,which is illustrated in FIG. 4.

Accordingly, a local change of the discharge opening in a certainposition normally influences the fibre direction in adjacent positions.Consequently, the control signal s_(n) to the actuating member in theposition n preferably should be a function of a predetermined number oferror values, preferably at least two error values, originating fromneighbouring positions, i.e. n−1, n+1, n−2, n+2 . . . .

The control unit 13 then calculates each control signal in accordancewith

$\begin{matrix}{s_{n} = {{e_{n}C_{0}} + {e_{n + 1}C_{1}} + {e_{n + 2}C_{2}} + \ldots + {e_{n + J}C_{J}} -}} \\{\left( {{e_{n - 1}C_{1}} + {e_{n - 2}C_{2}} + \ldots + {e_{n - J}C_{J}}} \right)} \\{{= {{e_{n}C_{0}} + {\sum\limits_{i = 1}^{i = J}\;{C_{i}\left( {e_{n + i} - e_{n - i}} \right)}}}},}\end{matrix}$where J is a predetermined integral number and C₀ C₁ C₂ . . . C_(J) arepredetermined constants. If J, for example, is selected to be 5, thecontrol signal s_(n) to the actuating member in the position nconsequently will be a function of both the error values in the positionn and in the adjacent positions n+1, n−1, n+2, n−2, n+3, n−3, n+4, n−4,n+5 and n−5. Accordingly, the constants define a filter having a widthwhich is determined by the choice of J.

In order to calculate the control signals to the J outermost actuatingmembers on each side, i.e. the actuating members in the positions n=1 ton=J and n=N−J to n=N, the dummy error values e_(−J+1) to e₀ and e_(N+1)to e_(N+J), which are set to be 0, are inserted.

Preferably, the control unit 13 includes a microprocessor (not shown)which performs the above-mentioned calculations. When the control unit13 has calculated the control signals, these are transmitted to theactuating members 14, preferably via a suitable regulator (not shown).

When performing trials in a machine for manufacturing board, a 50%reduction of fibre direction variations in the cross direction of thefibre web has been achieved by means of using different filtersaccording to the above-described method. Examples of such filters are:

-   -   C₀=0    -   C₁=0.0650    -   C₂=0.3150    -   C₃=0.5000    -   C₄=0.3150    -   C₅=0.0650    -   C₆=0        and    -   C₀=0    -   C₁=0.0326    -   C₂=0.1599    -   C₃=0.4144    -   C₄=0.7360    -   C₅=0.9670    -   C₆=0.9670    -   C₇=0.7360    -   C₈=0.4144    -   C₉=0.1599    -   C₁₀=0.0326    -   C₁₁=0.

In the first example is J=6, and in the second example is J=11.Alternatively, larger filters can be utilised, for example such whereJ=30 or even J=60. However, the filters are machine-specific and, evenif these filters have proven to function well in the board machine inquestion, it is evident that other filters may be preferable in otherpaper or board machines. In the examples above, all constants are equalto or larger than 0, which is preferred, but also negative values can beutilised for the constants. However, it is preferred that the constantC₀ is chosen to be 0 since, in accordance with the description given inconnection with FIGS. 3 and 4, as a rule, a correction of the fibredirection in the position n will not be promoted by a change of thedischarge opening in said position n.

In order to ensure that the fibre direction is kept within prescribedlimit values, it is preferred that the above-described steps, i.e.measurement of the fibre direction, calculation of appropriate controlsignals, and adjustment of the discharge opening in accordance withthese control signals, take place substantially continuously during thepaper manufacture. In practice, however, it takes a certain time for thefibre direction meter 12 to scan across the width of the fibre web 8when measuring the fibre direction, and therefore it may instead be morepractical to allow the discharge opening to change one to two times perminute, or with any other suitable time-interval.

It is evident that, within the scope of the invention, it is possible touse other algorithms than the one described above for calculatingappropriate control signals from the measured fibre direction values.For instance, the average of the error profile can be calculated andcorrected separately, or alternatively, the error profile can be dividedinto different wavebands which are treated separately, a technique whichis known per se. It is also possible to apply additional filters in thealgorithm, for example in order to reduce so-called “ringings” in thesystem.

It is also evident that the invention is applicable on different typesof paper as well as board machines, and that these machines can includea plurality of formers and headboxes, where the invention isimplemented.

1. A device for on-line control of the fibre direction of a continuous fibre web in a paper or board machine, comprising at least one former including at least one headbox being arranged for delivering a stock, which in said former is formed into said fibre web, though a slice including lips which are movable in relation to each other and define a discharge opening, said device including: a fibre direction meter arranged downstream the former for measuring the fibre direction of the fibre web; a predetermined number of actuating members, which are arranged in predetermined positions along said lips for regulating the discharge opening locally as a response to individual control signals, each being a function of measured fibre direction values; and a control unit, which is arranged for receiving the measured fibre direction values from the fibre direction meter, calculating said control signals, and transmitting the control signals to the actuating members, wherein the improvement comprises: the control unit is arranged for identifying an array of fibre direction values, {v₁ v₂ v₃ . . . v_(N)}, of the measured fibre direction values, said fibre direction values of said array originating from positions in the cross direction of the fibre web which correspond to the positions of the actuating members; the control unit is arranged for comparing the identified array of fibre direction values with an array of desired fibre direction values {b₁ b₂ b₃ . . . b_(N)} by means of calculating an array of error values, {e₁ e₂ e₃ . . . e_(N)}={b₁-v₁ b₂-v₂ b₃-v₃ . . . b_(N)-v_(N)}; the control unit is arranged for calculating said control signal for each actuating member as a function of a predetermined number of said error values in accordance with ${s_{n} = {{e_{n}C_{0}} + {\sum\limits_{i = 1}^{i = J}\;{C_{i}\left( {e_{n + i} - e_{n - i}} \right)}}}},$ where J is a predetermined integral number and C₀ C₁ C₂ . . . C_(J) are predetermined constants; and wherein said calculations are performed b a microprocessor included within said the control unit.
 2. A device according to claim 1, wherein the constants C₁ C₂ . . . C_(J) are larger than
 0. 3. A device according to claim 1, wherein C₀=0.
 4. A device according to claim 2, wherein C₀=0.
 5. A method for on-line control of the fibre direction of a continuous fibre web in a paper or board machine comprising at least one former including at least one headbox being arranged for delivering a stock, which in the former is formed into said fibre web, though a slice including lips which are movable in relation to each other and define a discharge opening, said method including: measuring the fibre direction of the fibre web by means of a fibre direction meter arranged downstream the former; calculating and transmitting individual control signals, each being a function of measured fibre direction values, to a predetermined number of actuating members, which are arranged in predetermined positions along the lips for regulating the discharge opening locally as a response to the control signals, wherein the improvement comprises: identifying an array of fibre direction values, {v₁ v₂ v₃ . . . v_(N)}, of the measured fibre direction values, said fibre direction values of said array originating from positions in the cross direction of the fibre web which correspond to the positions of the actuating members; comparing said identified array of fibre direction values with an array of desired fibre direction values, {b₁ b₂ b₃ . . . b_(N)}, by means of calculating an array of error values in accordance with {e₁ e₂ e₃ . . . e_(N)}={b₁-v₁ b₂-v₂ b₃-v₃ . . . b_(N)-v_(N)}; and calculating each of said control signals as a function of a predetermined number of said error values in accordance with ${s_{n} = {{e_{n}C_{0}} + {\sum\limits_{i = 1}^{i = J}\;{C_{i}\left( {e_{n + i} - e_{n - i}} \right)}}}},$ where J is a predetermined integral number and C₀ C₁ C₂ . . . C_(J) are predetermined constants.
 6. A method according to claim 5, wherein the constants C₁ C₂ . . . C_(J) are larger than
 0. 7. A method according to claim 5, wherein C₀=0.
 8. A method according to claim 6, wherein C₀=0.
 9. A device for on-line control of the fibre direction of a continuous fibre web in a paper or board machine, comprising at least one former including at least one headbox being arranged for delivering a stock, which in said former is formed into said fibre web, though a slice including lips which are movable in relation to each other and define a discharge opening, said device comprising: a fibre direction meter arranged downstream of the former for measuring the fibre direction of the fibre web; a predetermined number of actuating members, which are arranged in predetermined positions along said lips for regulating the discharge opening locally as a response to individual control signals, each being a function of measured fibre direction values; a control unit, said fibre direction meter and said actuating members being directly connected to said control unit, said control unit being arranged for receiving the measured fibre direction values from the fibre direction meter, calculating said control signals, and transmitting the control signals to the actuating members; the control unit is arranged for identifying an array of fibre direction values, {v₁ v₂ v₃ . . . v_(N)}, of the measured fibre direction values, said fibre direction values of said array originating from positions in the cross direction of the fibre web which correspond to the positions of the actuating members; the control unit is arranged for comparing the identified array of fibre direction values with an array of desired fibre direction values {b₁ b₂ b₃ . . . b_(N)} by means of calculating an array of error values, {e₁ e₂ e₃ . . . e_(N)}={b₁-v₁ b₂-v₂ b₃-v₃ . . . b_(N)-v_(N)}; and the control unit is arranged for calculating said control signal for each actuating member as a function of a predetermined number of said error values in accordance with ${s_{n} = {{e_{n}C_{0}} + {\sum\limits_{i = 1}^{i = J}\;{C_{i}\left( {e_{n + i} - e_{n - i}} \right)}}}},$ where J is a predetermined integral number and C₀ C₁ C₂ . . . C_(J) are predetermined constants. 